Asymmetric resource competition in a filter‐feeding stream insect (Hydropsyche siltalai; Trichoptera)

SUMMARY. 1. Field experiments in a lake outlet stream were carried out on filter‐feeding larvae of the caddisfly Hydropsyche siltalai Döhler, to assess whether microhabitat quality, establishment and growth rate is affected by position (upstream—downstream) within a dense aggregation of larvae. 2. Artificial substrates holding high‐density aggregations of hydropsychid larvae were used. Marked III‐instar H. siltalai larvae were implanted either in the upstream end or in the downstream end of an aggregation and recovered after 17 days. 3. Larvae recovered from the downstream end of an aggregation were smaller than those that had been implanted in the upstream end. 4. Fewer larvae had settled in the downstream end of aggregations compared to the exposed upstream part. 5. Naturally colonizing H. sitalai larvae were, in contrast to implanted larvae, more numerous behind aggregations. Small (first and second instar) Hydropsyche angustipennis (Curtis) and/or H. pellucidula (Curtis) larvae were more numerous in front of aggregations. 6. Current velocities, measured with a microcurrent velocity meter, and density of drifting prey, were significantly lower behind aggregations of hydropsychid larvae. Food availability decreased by about 50% behind aggregations.